Sora-Heartless is a minor character and a form that Sora takes during the final act of Kingdom Hearts. In Kingdom Hearts coded, the digital version of this Heartless serves as the main antagonist and the source of the Bug Blox corruption. Unlike other Heartless, Sora-Heartless held onto his feelings, memories, and selfhood as a Heartless, and this unique trait is commented on by others.
After Sora, Donald, and Goofy track Riku down to the Great Hall, they learn that he has used the incomplete Keyblade of People's Hearts to summon the Keyhole of Hollow Bastion. He reveals that Kairi's lost heart has been within Sora all along, and that it must be released in order to complete the Keyhole. He moves to release Kairi's heart by using the Keyblade on Sora, but Sora parries at the last second, and eventually defeats Riku.
Despite this, Kairi is still catatonic, and Sora is unable to seal the incomplete Keyhole. In order to save Kairi, Sora turns Riku's Keyblade on himself. This releases Kairi's heart from Sora, reviving her, but also completes the Keyhole and casts Sora into darkness, transforming him into a Heartless, and releasing his Nobody, Roxas, as well as Kairi's Nobody, Naminé. Sora fades into a Shadow, as he does not have that much darkness in his heart, just as Kairi awakens, and the Heartless rapidly emerging from the Keyhole force Kairi, Donald, and Goofy to flee.
As a Shadow, Sora follows his friends through the Lift Stop to the Entrance Hall, where he sees them being overrun by other Shadow Heartless. He rushes to help his friends, and though Donald tries to attack him, Kairi recognizes him and embraces him (in the manga, Kairi jokes that she recognized Sora's Heartless by the shape of its calves). Her light and heart return Sora to his human form, and Sora later states that while a Shadow, he nearly forgot everything. Sora remains a pseudo-Heartless throughout his battle with Ansem, Seeker of Darkness until he is reunited with his Nobody a year later.
Kingdom Hearts: Chain of Memories
When they first enter Castle Oblivion and learn of its alleged power over memories, Goofy reassures Sora by reminding him that he didn't lose his memories when he became a Heartless. Axel later explains to Larxene that this strange trait is what interests him about Sora. Sora and his friends eventually discover that the witch Naminé somehow has the ability to manipulate memories, allowing her to manipulate the memories of Sora and his friends.
Organization XIII are forcing her to do this in order to brainwash Sora at Castle Oblivion, but Sora defeats the Organization members within the castle and rescues Naminé. They agree that Sora will be put to sleep so that Naminé can restore his jumbled memories, and after Naminé puts Sora to sleep, she blanks the contents of [[Jiminy's Journal], as well as Sora and his friends' memories, then begins restoring them. However, while the memories were reconnected, the Journal itself remained blank. This is later revealed to be due to the record of Sora-Heartless beginning to assert itself.
Kingdom Hearts II
One year later, the Replica of Sora's Nobody, Xion, is reunited with his Nobody, Roxas, and the pivotal creation of Sora's Heartless later appears within Roxas's final dream-memories. Finally, Roxas reunites with Sora, and Sora reawakens as a complete being, no longer a Heartless. Although he is whole, his time as a Heartless and the magic of his new clothes allows him to infrequently assume the Heartless-based Anti Form.
Kingdom Hearts coded
As a symptom of the secrets that were hidden within the journal, the digital recording of Sora's Heartless gained new life and spawned the Bug Blox which spread throughout the Data Worlds.
After clearing the Bug Blox from the data Hollow Bastion, the Journal reveals to Data Sora that the digitized version of Sora-Heartless was the source of the Bug Blox corrupting the Journal. While Data Sora had been exploring the Data Worlds and gained new power, the digitized Sora-Heartless was corrupting the other data, changing them to make itself stronger.
Data Sora attacks the digitized Sora-Heartless, but instead of releasing hearts it releases the minds of slain Heartless, which it reabsorbs, increasing its power exponentially. Data Sora attacks it again as it takes on the form of Anti-Sora, and just as the Heartless is about to win the fight, King Mickey appears and helps Data Sora slay it.